In worm presses or extruders, the medium (e.g. a plastified synthetic resin) is formed by a worm shaft through a gap defined between a constriction plate surrounding the worm shaft and a counter element arranged on the latter. In order to provide desired flow resistance values, the dimensions of the gap can be varied. The use of these constrictions are required to maintain the medium at a certain pressure and temperature in degasification, homogenizing and/or plastifying zones and for feeding the thermoplastic to implements located at the outlet of the extruder, e.g. the extrusion die or an injection or blow molding head, for different media, or for different operating temperatures, or different speeds of the worm shaft.
The German published specification DE-AS No. 15 54 751 describes the use of a plurality of constriction plates each providing a different diameter of the gap so as to control the flow of the medium i.e. the resistance impeding the flow of the medium. The German published specification DE-AS No. 21 55 855 illustrates in a similar manner the replacement of constriction elements or counter elements with different diameters.
From this prior art it is known that the gap can be varied in order to provide desired pressures or temperatures of the medium. However, these systems have the disadvantage of requiring disassembly of the worm press for replacement of the constriction plate or counter element, respectively, each time a different pressure is required or when the medium has a different temperature or composition. The disassembly of the worm press requires substantial work and above all causes a stoppage of the process.
In order to provide a continuous adjustment of the gap during operation of the worm pressing, the German utility model DE-GM No. 81 18 814 describes a conical constriction plate which is shifted toward a counter element on the worm shaft by means of a drive mechanism. Consequently, the worm press does not require disassembly for adjusting the gap. However, it has the drawback that the parts which are shifted relative to one another are subjected to a considerable stress by the contraction pressure and thus are prestressed by relatively heavy forces. For the drive mechanism, therefore, high performance motors are required via reduction gears to overcome these forces. Thus, the adjustment is cumbersome.
A further drawback of this system is that the adjustment, that is the displacement of the constriction plate, is only possible over a short length which is not sufficient even when using highly viscous grease for lubrication or where the lubricating film is drawn toward the areas supporting each other. Therefore considerable wear is obtained which can be overcome only by utilizing enhanced lubricating systems.